JPH058122A - Wire electric discharge machining apparatus - Google Patents

Abstract

PURPOSE:To provide a wire erectric discharge machining apparatus for preventing the occurrence of a drum-like shape peculiar to wire electric discharge machining and improving accuracy in straight working and a corner part working. CONSTITUTION:This apparatus is provided wire a working power supply 8 for suppling electric discharge current, a voltage detecting circuit 9 for detecting average voltage between electric discharge electrodes in working, a current detecting circuit 14 for detecting average current and a controller 10 for controlling the electric discharge voltage and current. When working feed speed is controlled to keep an average voltage value constant, the controller 10 controls an off-time, pulse cycle or the like of electric discharge current to change the average current in inverse proportion to the working feed speed and when the controller controls the working feed speed to keep the average current value constant, the controller 10 controls the off-time of electric discharge voltage, pulse cycle or the like to change the average voltage in proportion to the working feed speed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highly accurate wire electric discharge machine. In particular, by controlling so as to cancel the electrostatic repulsion force and the electrostatic repulsion force generated between the electrode and the work piece, the phenomenon that the work shape of the work piece that is peculiar to wire electric discharge machining becomes a drum shape occurs. The present invention relates to a wire electric discharge machine for preventing the wire discharge.

[0002]

2. Description of the Related Art In a conventional wire electric discharge machining apparatus, a phenomenon occurs in which a machined shape of a work piece becomes a drum shape, especially in finishing, which deteriorates machining accuracy. This is a phenomenon peculiar to wire electric discharge machining, and was a very serious problem particularly in press die machining. The drum shape has a concave shape as shown in FIG. 9A and a convex shape as shown in FIG. 9B. The drum shape and the amount of curvature are variously changed depending on the processing conditions, the processed plate thickness and the like. It is empirically known.

The structure and operation of a conventional wire electric discharge machine will be described below with reference to the drawings. FIG. 8 is a diagram showing a configuration of a conventional wire electric discharge machine. In the figure, 1 is a wire electrode for forming an electrode for electric discharge machining, 2 is a workpiece to be electric discharge machined, 3 is a wire bobbin for supplying a wire electrode, 4a and 4b are discharge gaps from the upper and lower portions, respectively. A machining liquid nozzle for supplying a machining liquid to a portion, 5 is an electric power feeder for supplying electric power to the wire electrode 1, 6 is a tension roller for applying a tension to the wire electrode 1, 7 is a processed wire electrode collection box, 8 is a wire electrode and a cover. A machining power supply that supplies a machining current that flows between the discharge electrodes and the workpiece, 9 is a voltage detection circuit that detects the average voltage between the discharge electrodes, and 10 is a machining feed speed calculated from the result of the voltage detection circuit. A control device for controlling, 11 is a servo amplifier for driving the drive motor, 12 is a drive motor for moving the workpiece 2 fixed on an XY cross table (not shown) in the XY directions. A.

Next, the operation will be described. In FIG. 8, the wire electrode 1 runs in a state where tension is applied by the tension roller 6, and a machining current is supplied to the wire electrode 1 from the machining power source 8 through the power supply 5. A discharge gap is formed between the workpiece 2 and the wire electrode 1. Pure water, which is a working liquid, is supplied to the discharge gap from above and below through the working liquid nozzles 4a and 4b, and an electric discharge is generated in the discharge gap to process the workpiece. In the control of the machining feed, the average voltage V during machining is detected by the voltage detection circuit 9 connected to the discharge electrode, and the detected value is controlled to be equal to a preset voltage.

That is, when the average voltage in the discharge gap is higher than the set voltage value, the discharge gap is in a large state. Therefore, the control device 10 causes the drive motor 12 to increase the machining feed speed. Issue a command to. In response to this command, the controller 10 increases the machining feed speed of the drive motor 12 and controls the discharge gap to be narrowed. On the contrary, when the average voltage in the discharge gap is lower than the set voltage value, the discharge gap is in a small state, and therefore the control device 10 controls the servo amplifier 11 so that the drive motor 12 reduces the machining feed speed. Issue a command. In response to this command, the control device 10 reduces the machining feed rate of the drive motor 12 to control the discharge gap to be wide. That is, the control device 10 always controls the discharge gap to be the set value. FIG. 11 is a diagram showing the relationship between the discharge voltage and the discharge current. As shown in the figure, when a high voltage is applied for a time t ₁ (no load time) before the discharge is started, and then the insulation in the discharge gap is broken, a discharge current flows for a time t ₂ and then The discharge is stopped for t ₃ (off time). The cycle T of the discharge voltage has a relationship of T = t ₁ + t ₂ + t ₃ . The time (t ₁ + t ₂ ) is the time (on-time) during which the discharge voltage is applied. In this figure, the average voltage V is a value obtained by averaging the voltage waveform over time.

[0006] Generally, after roughing (first cutting), highly precise finishing (second cutting) is performed. At that time, as described above, a drum-shaped error peculiar to wire electric discharge machining occurs in the workpiece after finishing. Figure 1
0 shows the measurement result of a typical drum shape in the finishing process. The straight line in the figure is the predicted value when there is no error, and the broken line is the measured value. As described above, it can be seen that the machining error increases in the central portion of the workpiece. Regarding the cause of the drum shape, the conventional idea is that it is due to the vibration of the wire electrode (first-order mode), and the machining liquid is jetted from above and below the surface of the workpiece in normal machining, so the central part of the discharge gap is Although there was an idea that the specific resistance was lowered by ionization and the drum shape was generated as a result, there was no effective method for suppressing the generation of the drum shape. On the other hand, as to the problem that the wire electrode is bent by the electric discharge repulsive force during the rough machining and the machining accuracy is deteriorated, as shown in JP-B-3-29531, the electrostatic attraction force is generated between the wire electrode and the workpiece. Alternatively, a method of intentionally generating an electromagnetic attraction force to cancel the discharge repulsion force has been proposed, but it was necessary to provide a compensation signal circuit separately from the machining power supply circuit.

[0007]

In the conventional wire electric discharge machining apparatus, the cutting surface of the workpiece after machining has a drum shape especially in finishing, which deteriorates the machining accuracy. However, this drum shape is generated. There was no effective way to prevent. Further, as described above, the drum shape has various shapes and bending amounts depending on the processing conditions, the processed plate thickness, and the like.
Therefore, when the plate thickness of the work piece changes during processing, the drum shape is likely to occur due to the change in processing conditions, and it has been extremely difficult to maintain straightness accuracy. In addition, as in Japanese Patent Publication No. 3-29513, by adding a compensation signal between the upper and lower sides of the electrode or between the electrode and the workpiece, the deflection of the wire electrode during rough machining could be reduced, but the electrostatic force, Although the suction force such as electromagnetic force can be controlled in the direction of increasing it, but it cannot be controlled in the direction of decreasing it, it is impossible to reduce the concave drum shape generated in the finishing process.

The present invention has been made in order to solve such a conventional problem, and controls so that the repulsive force generated by electric discharge between the wire electrode and the workpiece and the attractive force due to the electrostatic force cancel each other out. Therefore, it is an object of the present invention to provide a wire electric discharge machine that prevents the occurrence of a drum shape that is peculiar to wire electric discharge machining.

[0009]

DISCLOSURE OF THE INVENTION The present invention provides a wire electric discharge machine for processing a workpiece by discharging in a discharge gap formed by a wire electrode and the workpiece.
A processing power supply that supplies a discharge current, a voltage detection circuit that detects an average voltage between discharge electrodes, a current detection circuit that detects an average current of the discharge current, and a control device that controls the discharge current or the discharge voltage, When controlling the machining feed speed so that the average voltage value detected by the voltage detection circuit becomes constant, this control device decreases the average current of the current detection circuit as the machining feed speed increases, and the machining feed speed When the machining feed speed is controlled so that the average current of the current detection circuit increases according to the decrease and the average current value detected by the current detection circuit becomes constant, the average of the voltage detection circuit increases as the machining feed speed increases. The voltage is increased and the average voltage of the voltage detection circuit is decreased as the machining feed rate is decreased.

[0010]

In the wire electric discharge machining apparatus according to the present invention, when the machining feed speed is controlled so that the average voltage value detected by the voltage detection circuit becomes constant, the average current of the current detection circuit increases as the machining feed speed increases. To control the control device so as to increase the average current of the current detection circuit as the machining feed speed decreases, and control the machining feed speed so that the average current value detected by the current detection circuit becomes constant. Is generated by discharge by controlling the control device so that the average voltage of the voltage detection circuit increases as the machining feed speed increases, and the average voltage of the voltage detection circuit decreases as the machining feed speed decreases. The discharge repulsive force generated and the electrostatic attraction force between the electrode and the work piece are canceled to keep the wire electrode in a straight state during processing and suppress the generation of drum shape.

[0011]

EXAMPLES Prior to the description of the examples, the cause of the drum shape will be considered. As described above, conventionally, there is an idea that the cause of the drum shape is vibration of the wire electrode (first mode) and a decrease in the specific resistance of the central portion of the discharge gap. The following is a list of questions in the conventional theory. (1) In the case of the wire vibration theory (a) As a result of measurement, vibration of the wire electrode in the lower order mode was observed in low-speed machining with a wide discharge gap. However, in stable finishing, the wire vibration is random and the amplitude is quite small. (B) It is difficult to explain the generation process of the convex inverted drum shape that occurs in finishing processing at a relatively high speed. (2) In the case of the specific resistance theory (a) Even if kerosene is used as the working fluid, the drum shape can be ignored. (B) The drum shape is generated even in processing with a high-frequency AC power supply in which electrolysis is not generated. Therefore, as a cause of the drum shape, there is a situation that cannot be explained by the conventional way of thinking. Recent research (VDI-Zeitschrift, German Institute of Technology, 1976, 11
Vol. 8, pp. 13-17) point out that there are electrolysis, electromagnetic field, spark discharge, and dielectric bubbles in the force acting on the electrode during electric discharge machining. It has been obtained that the electrostatic force generated between the electrode and the workpiece largely contributes to the drum shape. Hereinafter, that point will be described.

FIG. 2 is a graph showing the relationship between the average voltage and the maximum bending amount of the drum shape. From the figure, it can be seen that there is a substantially linear relationship between the average voltage and the amount of drum-shaped curvature. Particularly, in the processing (A) in which the average current I is small, the concave error increases as the average voltage V increases. This drum-shaped curve is considered to be due to the electrostatic attraction force acting between the wire electrode and the workpiece. First, the electrostatic attraction force will be considered. FIG. 3 is a model diagram for calculating the electrostatic attraction force. Per unit length electrostatic attraction f ₁ in the discharge gap by an electrical imaging in FIG. _{3, f 1 = (1/2)} πε 0 V 2/2 (r + e) · {log [2 (r + e) / r ] (1) where ε ₀ : dielectric constant, V: voltage between discharge electrodes, r: radius of wire electrode, e: discharge gap

FIG. 4 is a model diagram for calculating the deflection δ of the wire when an external force acts on the wire. Generally, as shown in FIG. 4, when the external force f (force per unit length) acts, the wire deflection δ is δ = f · h ² / 8T (2) where h: plate of workpiece Thickness, T: Wire tension

The deflection amount δ ₁ of the wire electrode due to the electrostatic attraction force f ₁ calculated by substituting f ₁ in the equation (1) into f in the equation (2)
Is shown by the dotted line in FIG. From the figure, the white circles, black circles, white squares, and the solid line connecting these indicate the measured values of the drum volume when the average current was changed. It can be seen that the drum amount (A) in the processing with a small average current I substantially matches the calculated wire deflection amount δ ₁ (dotted line). Further, as the average current I increases (A → B → C), the straight line shifts in the negative direction (the direction in which the concave depression decreases). However, the slope of the straight line is constant and equal to the slope of the electrostatic attraction force (dotted line) calculated. This is an electrostatic attraction force f ₁ to the wire electrode and acts opposite direction of the discharge repulsive force f _2, the electrostatic attraction force f ₁ increases the discharge repulsive force f ₂ as the average current I increases It is thought to be due to the cancellation.

[0015] the discharge repulsive force f ₂ is by increasing the average current I (A → B → C) , the discharge repulsive force f ₂ acts made to cancel the electrostatic attraction force f ₁ increases. From this, it can be considered that the discharge repulsion force f ₂ is almost proportional to the average current I. That is, the discharge repulsion force f ₂ can be expressed as follows. f ₂ = kI (3) Here, k is a proportional constant. That is, in the finishing process, the wire electrode is subjected to the opposite force of the discharge repulsion force f ₂ and the electrostatic attraction force f _{1, and} the wire electrode is deflected by the combination of these two forces to determine the drum shape. It is speculated that it has been done.

As a result of repeated experiments under various conditions based on the above assumptions, as described above, the wire electrode being processed has an electrostatic attraction force f ₁ proportional to the average voltage and an average current. It has been revealed that the discharge repulsive force f ₂ that is almost proportional to the force acts, and the wire electrode is deflected by the combined force of these two forces to form a drum shape. From this, it is possible to prevent the deformation of the wire electrode and suppress the generation of the drum shape by controlling the electrostatic attraction force f ₁ and the discharge repulsion force f ₂ by some method. There was found.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention, in which 1 is a wire electrode, 2 is a workpiece, 3 is a wire bobbin, 4a and 4b are upper and lower working fluid nozzles, respectively.
Reference numeral 5 is a power supply for supplying power to the wire electrode 1, 6 is a tension roller for applying tension to the wire electrode 1, 7 is a processed wire electrode collection box, 8 is a processing power supply for supplying a discharge current to the discharge gap, and 9 is a discharge electrode. Voltage detection circuit for detecting the average voltage between the electrodes, 13 a current probe for detecting the discharge current, 14 a current detection circuit for detecting the average current between the discharge electrodes, 10 a machining feed rate from the result of the voltage detection circuit To control the machining feed rate and control the off time of the machining pulse, the pulse cycle, etc. so that the average machining current changes in inverse proportion to the machining feed rate, 11 servo amplifiers, 12 are not shown XY cross table It is a drive motor that moves the workpiece 2 fixed to the XY direction.

Next, the operation will be described. Similar to the conventional example, the wire electrode 1 travels in a state where tension is applied by the tension roller 6, and a discharge current is supplied to the wire electrode 1 from the machining power source 8 through the power supply 5. Pure water, which is a working liquid, is supplied to the discharge gap formed by the work piece 2 and the wire electrode 1 from above and below through the working liquid nozzles 4a and 4b, and an electric discharge is generated in the discharge gap to generate the work piece. Processing is performed.

(1) When the average voltage is controlled to be constant In controlling the machining feed, the average voltage during machining is detected by the voltage detection circuit 9 connected between the discharge electrodes, and this detected value is set in advance. Is controlled to be equal to the set voltage.
That is, when the average voltage between the discharge electrodes is higher than the set voltage value, the discharge gap becomes large. Therefore, the control device 10 issues a command to the servo amplifier 11, and the drive motor 12 increases the machining feed speed. Let On the contrary, when the average voltage between the discharge electrodes is lower than the set voltage value, the discharge gap becomes small, so the control device 10 issues a command to the servo amplifier 11 to drive the drive motor 12.
Reduces the machining feed rate. FIG. 12 is a diagram showing the relationship between the discharge voltage waveform and the discharge current waveform when the average voltage is controlled to be constant. As shown in the figure, when a high voltage is applied for a time t ₁ (no load time) before the discharge is started, and then the insulation in the discharge gap is broken, a discharge current flows for a time t ₂ and then The discharge is stopped for t ₃ (off time). The cycle T of the discharge voltage is T = t ₁ + t ₂ + t ₃
Have a relationship. The time (t ₁ + t ₂ ) is the time (on-time) during which the discharge voltage is applied. In this figure, the average voltage V is a value obtained by averaging the voltage waveform over time. The method for obtaining the average value of the discharge voltage is a known technique, and there are a method of changing the ratio of the on time and the off time of the pulse, a method of changing the pulse period, a method of changing the pulse amplitude, and the like. In the figure, the average voltage is controlled to be constant by changing the pulse period and the ratio of on-time and off-time. It can be understood that the average current is increased by shortening the cycle of the discharge voltage waveform. In the present invention, the voltage detection circuit 9 not only detects the average value of the discharge voltage, but also detects the integrated value of the discharge voltage and controls the integrated value to be equal to a preset value. You can also

(2) When the average current is controlled to be constant In controlling the machining feed, the average current during machining is detected by the current detection circuit 14 connected between the discharge electrodes, and this detected value is preset. The current is controlled to be equal to the set current. That is, when the average current between the discharge electrodes is lower than the set current value, the discharge gap becomes large and the discharge occurrence frequency (discharge frequency) is lowered. Therefore, the controller 10 issues a command to the servo amplifier 11. , Drive motor 1
2 increases the machining feed rate. On the contrary, when the average current between the discharge electrodes is higher than the set current value, the discharge gap becomes small and the frequency of discharge occurrence increases, so the controller 10 issues a command to the servo amplifier 11 to drive the servo amplifier 11. The motor 12 reduces the machining feed rate. FIG. 13 is a diagram showing the relationship between the discharge current waveform and the discharge voltage waveform when the average current is controlled to be constant. As shown in the figure,
By making the pulse period constant, the average value of the discharge current can be made constant. In this case, in order to change the average value of the discharge voltage, it can be achieved by lengthening the no-load time t ₁ . In the figure, it can be easily understood that the average voltage can be increased by increasing the no-load time t ₁ to t ₁ ′ and shortening the off time t ₃ to t ₃ ′. In the present invention, the current detection circuit 14 not only detects the average value of the discharge current, but also detects the integrated value of the discharge current and controls the integrated value to be equal to a preset set value. You can also

Normally, the processing start condition is selected such that the drum shape does not occur, but when the plate thickness of the workpiece changes during processing, electrostatic attraction force f ₁ and discharge repulsion force f ₂ The balance is lost and the wire electrode is bent. For example, if the plate thickness of the work piece is doubled during processing, the electrostatic attraction force f ₁ per unit length does not change, but from equation (2),
Since the amount of deflection δ ₁ due to the electrostatic attraction force is proportional to the square of the plate thickness, δ ₁ = [f ₁ (2h) ² ] / 8T = 4 · f ₁ · h ² / 8T = 4δ ... ( 3) and 4 times. Meanwhile, since the thickness in the case of the average current constant discharge repulsive force f ₂ per unit length becomes twice is 1/2, the amount of deflection [delta] ₂ by the discharge repulsive force f ₂ is similarly (2) From the equation, δ ₂ = (1/2) f ₂ · (2h) ² / 8T = 2f ₂ · h ² / 8T = 2δ ··· (4) Therefore, δ ₁ = 2δ ₂ and electrostatic attraction The deflection δ ₁ due to the force is much larger than the deflection δ ₂ due to the discharge repulsion force. As a result, the wire electrode is attracted to the processed surface side of the work piece to be bent, and the processed work piece has a concave drum shape. vice versa,
When the plate thickness of the work piece becomes smaller during processing, the deflection δ ₁ due to the electrostatic attraction force decreases. When the electric discharge repulsive force becomes larger than the electrostatic attraction force, the shape of the processed workpiece becomes a convex drum shape. In this embodiment, by varying the average current or the average voltage, the discharge repulsion force f ₂ or the electric attraction force f ₁
Is controlled to maintain the balance between the electrostatic attraction force and the discharge repulsion force.

(1) When the average voltage is controlled to be constant When the average voltage is controlled to be constant, a change in the plate thickness of the workpiece during machining appears as a change in the machining feed rate. That is, for example, when the plate thickness of the work piece is doubled as in the previous example, the processing speed is halved with the constant average voltage control. The control device 10 recognizes the change in the plate thickness of the workpiece from the change in the machining speed, and controls the off time of the machining pulse so that the average machining current is inversely proportional to the machining feed speed, that is, until the average current doubles. The processing power supply 8 is controlled so as to reduce the size. Since the total discharge repulsive force on the wire electrode is proportional to the average current, doubling the average current also doubles the total discharge repulsive force. On the other hand, since the plate thickness of the work piece is doubled, the discharge repulsion force per unit length does not change, but the amount of deflection δ ₂ 'can be calculated from equation (2) as δ ₂ ' = f ₂ · (2h ) ² / 8T = 4 · f ₂ · h ² / 8T = 4δ (5) Therefore, δ ₁ = δ ₂ 'becomes equal to the amount of deflection due to the electrostatic attraction force and the discharge repulsion force. The balance will be maintained. The average current flowing between the discharge electrodes is detected by the current probe 13 by the current detection circuit 14,
The value is controlled by the off time of the discharge current (machining current), the pulse cycle, or the like.

FIG. 5 shows the operating state of each part of the wire electric discharge machine when the plate thickness changes during machining when the average voltage is controlled to be constant. The horizontal axis of FIG. 5 shows the passage of time. In FIG. 5, (a) shows the temporal change of the plate thickness of the workpiece. (B) A state in which the average voltage V is controlled to be constant is shown. (C) shows the time change of the machining feed rate. It shows how the control is performed so as to be inversely proportional to the plate thickness of the workpiece. (D) shows the time change of the average current I. It shows how control is performed so as to be proportional to the plate thickness of the workpiece. (E) shows the change over time in the amount of wire deflection δ ₁ due to the electrostatic attraction force. It shows how it changes in proportion to the plate thickness of the workpiece. (F) shows the time variation of the wire deflection amount δ ₂ due to the discharge repulsion force. It shows how it changes in proportion to the plate thickness of the workpiece. (G) A state in which the amount of bending of the wire due to the electrostatic attraction force and the amount of bending of the wire due to the repulsive force of the discharge are canceled out and the amount of bending Δ of the wire becomes 0 is shown.

(2) When the average current is controlled to be constant When the average current is controlled to be constant, a change in the work sheet thickness during processing appears as a change in the processing feed rate. That is, for example, as in the previous example, when the workpiece plate thickness is 2
If it doubles, the machining speed will be 1 /
Doubled. The control device 10 recognizes the change in the work sheet thickness from the change in the machining speed, and sets the off time of the machining pulse so that the average voltage is proportional to the machining feed speed, that is, until the average voltage becomes 1/2 times. The processing power source 8 is controlled so as to increase. Since the electrostatic attraction force per unit length is almost proportional to the average voltage (dotted line in FIG. 2), the average voltage becomes 1/2 times, and the electrostatic attraction force f ₁ per unit length is also 1.
/ 2 times, but since the plate thickness of the work piece is doubled, the amount of deflection δ ₁ 'can be calculated from equation (2) as follows: δ ₁ ' = 1/2 · f ₁ · (2h) ² / 8T = 2 · f ₁ · h ² / 8T = 2δ (5) Therefore, δ ₁ '= δ ₂ and electrostatic attraction force and discharge repulsion force become equal, and the balance between both is maintained. become. The average voltage between the discharge electrodes is detected by the voltage detection circuit 9, and its value is controlled by the off time of the discharge voltage or the pulse period.

FIG. 6 shows an operating state when the plate thickness changes during processing when the average current is controlled to be constant. The horizontal axis of FIG. 6 shows the passage of time. In FIG. 6, (a) shows the temporal change of the plate thickness of the workpiece. (B) A state in which the average current I is controlled to be constant is shown. (C) shows the time change of the machining feed rate. It shows how the control is performed so as to be inversely proportional to the plate thickness of the workpiece. (D) shows the time change of the average voltage V. It shows how control is performed so as to be proportional to the plate thickness of the workpiece. (E) shows the change over time in the amount of wire deflection δ ₁ due to the electrostatic attraction force. It shows how it changes in proportion to the plate thickness of the workpiece. (F) shows the time variation of δ ₂ of the amount of wire deflection caused by the discharge repulsive force. It shows how it changes in proportion to the plate thickness of the workpiece. (G) A state in which the amount of wire deflection δ ₁ due to the electrostatic attraction force and the amount of wire deflection δ ₂ due to the discharge repulsion force are canceled out and the amount of wire deflection Δ becomes zero. By performing the control described above during machining, the electrostatic attraction force and discharge repulsion force when the plate thickness changes during machining are canceled, so the balance between electrostatic attraction force and discharge repulsion force is maintained. Therefore, the machined surface of the workpiece is always linear, and high straightness accuracy can be obtained.

Further, in the embodiment in which the average voltage is controlled to be constant, the shape accuracy in the corner portion is greatly improved. The operation in corner finishing will be described below. First, in the processing at the out-corner portion as shown in FIG. 7A, the processing area is small, so that the processing speed is increased. According to this embodiment,
At that time, the machining current is reduced and the off-time is controlled to be long, so that the corner sag at the out-corner is reduced. On the contrary, in the in-corner portion as shown in FIG. 7B, the processing area becomes large, and the processing speed decreases. According to the present embodiment, at that time, control is performed so as to increase the machining current and shorten the off-time, so that the undercut (leftover) at the in-corner is reduced.

In the above embodiment, an example of controlling the average voltage value so as to be proportional to the machining feed speed and an example of controlling the average current value so as to be inversely proportional to the machining feed speed have been shown. The average voltage value control that is proportional to the speed and the average current value control that is inversely proportional to the machining feed speed may not be used, and the average voltage and the average current may be controlled at an appropriate ratio. Also, instead of a current probe, a shunt,
Other current detection means such as a resistor may be provided.

[0028]

As described above, according to the present invention, when the average voltage is controlled to be constant, the average current is controlled,
When the average current is controlled to be constant, the average voltage is controlled, so the electrostatic attraction force and the discharge repulsion force are controlled to always cancel each other, and the drum shape that is characteristic of wire electrical discharge machining is prevented. However, the processing accuracy in the straight direction is improved.
Further, even when the plate thickness of the workpiece changes during processing, the drum shape can be prevented from occurring.

Further, even in finishing such as a corner portion, since the average current is controlled according to the change of the machining speed due to the change of the area, the corner dripping and the corner left-out are eliminated, and the corner shape error is greatly reduced. The machining accuracy at the corner can be greatly improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a block of a configuration of a wire electric discharge machine which is an embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the average voltage and the maximum amount of drum-shaped curve (drum amount).

FIG. 3 is a model diagram for explaining an electrostatic attraction force acting between a wire electrode and a workpiece.

FIG. 4 is a diagram for explaining the deflection of the wire.

FIG. 5 is a diagram showing operating characteristics of each part of the wire electric discharge machining apparatus when the plate thickness changes during machining when the average voltage is controlled to be constant.

FIG. 6 is a diagram showing operating characteristics of each part of the wire electric discharge machining apparatus when the plate thickness changes during machining when the average current is controlled to be constant.

FIG. 7 is an explanatory diagram of a corner finishing process.

FIG. 8 is a diagram showing a configuration of a conventional wire electric discharge machine.

FIG. 9 is an explanatory diagram of a drum shape.

FIG. 10 is a diagram showing an actual measurement result of a drum shape in finishing processing.

FIG. 11 is a diagram showing a relationship between a discharge voltage waveform and a discharge current waveform at the time of discharging in the conventional technique.

FIG. 12 is a diagram showing a relationship between a discharge voltage waveform and a discharge current waveform when the average voltage is controlled to be constant.

FIG. 13 is a diagram showing a relationship between a discharge current waveform and a discharge voltage waveform when the average current is controlled to be constant.

[Explanation of symbols]

1 wire electrode 2 Workpiece 3 wire bobbins 4a, 4b Upper and lower machining fluid nozzles 5 power supply 6 Tension roller 7 wire electrode collection box 8 Processing power supply 9 Voltage detection circuit 10 Control device 11 Servo amplifier 12 Drive motor 13 Current probe 14 Current detection circuit

Claims (4)

[Claims] 1. A wire electric discharge machining apparatus for machining a workpiece by discharging in a discharge gap formed between a wire electrode and the workpiece, a machining power supply for supplying a discharge current, and a discharge electrode. A voltage detection circuit for detecting the average voltage, a current detection circuit for detecting the average current of the discharge current, and a control device for controlling the discharge current are provided so that the average voltage value detected by the voltage detection circuit becomes constant. Controlling the machining feed speed, and controlling the controller so as to decrease the average current of the current detection circuit as the machining feed speed increases and increase the average current of the current detection circuit as the machining feed speed decreases. A wire electric discharge machine characterized by the above. 2. The wire electric discharge machine according to claim 1, wherein the controller is controlled so that the average current changes in inverse proportion to the machining feed rate. 3. A wire electrical discharge machining apparatus for machining a workpiece by discharging in a discharge gap formed between the wire electrode and the workpiece, a machining power supply for supplying a discharge current, and a discharge electrode between the discharge electrodes. A voltage detection circuit for detecting the average voltage, a current detection circuit for detecting the average current of the discharge current, and a control device for controlling the discharge voltage are provided so that the average current value detected by the current detection circuit becomes constant. The control so as to increase the average voltage of the voltage detection circuit as the processing feed speed increases and decrease the average voltage of the voltage detection circuit as the processing feed speed decreases. A wire electric discharge machine characterized by controlling the machine. 4. The control device is controlled so that the average voltage changes in proportion to the machining feed rate.
The wire electric discharge machine described.